1. Field of the Invention
The present invention relates generally to aromatic polycyanate compounds which comprise cycloaliphatic moieties and mixtures of such compounds, a process for the production thereof, and to resins and thermoset products which are based on these compounds.
2. Discussion of Background Information
The cyanate derivatives of polyfunctional phenols have been prized for their combination of high glass transition temperature (Tg), high use temperatures, and good dielectric properties. The polyfunctional phenols are typically prepared by condensation of formaldehyde with phenols. This is a versatile method for producing polyphenolic compounds (or novolacs) that have many applications, such as epoxy hardeners and starting materials to make high performance epoxy resins. One undesirable feature of this chemistry is that there are many products formed from this condensation with a broad distribution of molecular weights. The product distribution is a function of the starting molar ratio of formaldehyde to phenol, catalyst, and other process parameters.
In order to produce a product with high functionality (at least 4 phenols per molecule on average) a material with a high polydispersity (Mw/Mn) inevitably results. Typical polydispersities of 3 to 6 are obtained. As a consequence, high functionality products typically have high molecular weights, and therefore the viscosities are high. High viscosity is a disadvantage in many applications such as casting, coating, and adhesive bonding.
Another example of a high functionality novolac is bisphenol A novolac (BPAN) made from bisphenol A and formaldehyde. Again, in order to obtain high functionality, high polydispersities (typically 2.5-5.5) and high viscosities must be tolerated. In the case of BPAN, it is difficult to remove unreacted bisphenol A, and therefore commercially available BPAN's are typically contaminated with ˜20 wt % bisphenol A. Conversion of these high functionality novolacs to cyanate esters therefore gives a high viscosity material. The reaction of bisphenol A and formaldehyde and the conversion of the resultant mixture of polyphenolic compounds into a mixture of polycyanate compounds can schematically be represented as follows:
